Information on 'Short Bolting'
Short bolting is a term frequently used to describe the situation when a bolt is installed and the thread does not fully protrude through the nut.
In order that the full strength of the nut is to be realised, it is vital that the bolt thread protrude through the nut. Failure to do so runs the risk that thread stripping will occur.
Typically the first few pitches of the thread can only be partially formed because of a chamfer etc. Standards such as ISO 4014 (Hexagon head bolts Product grades A and B) state that incomplete threads may be present up to two pitches from the thread end. Hence, it is common practice to specify that two thread pitches must protrude so that the full strength of the thread can be achieved. Having the end of the bolt flush with the nut runs the risk that two pitches may not be fully effective and hence increase the risk of thread stripping.
Nut thickness standards have been drawn up on the basis that the bolt will normally sustain tensile fracture before the nut or bolt thread will strip. If the bolt breaks on tightening, it is obvious that a replacement is required. Thread stripping tends to be gradual in nature. If the thread stripping mode can occur, assemblies may enter into service which are partially failed, this may have disastrous consequences. Hence, the potential of thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved. When specifying nuts and bolts it must always be ensured that the appropriate grade of nut is matched to the bolt grade. Importantly, if it is found that bolts are occasionally breaking at the assembly stage, this should be investigated and the torque adjusted. The risk is that thread stripping may also be present as a result of nuts being designed on a probability of failure rather than an absolute certainty that the bolt breaks before the thread strips.
In cases of when a threaded fastener is tapped into a plate or a block it is usually the case that the fastener and block materials will be of different strengths. If the criteria is adopted that the bolt must sustain tensile fracture before the female thread strips, the length of thread engagement required can be excessive and can become unrealistic for low strength plate/block materials. Tolerances and pitch errors between the threads can make the engagement of long threads problematical without thread binding and seizure occurring.
In summary the full height of the nut is to be used if you are to avoid thread stripping - that is - ensure that the thread protrudes through the nut. Also use a standard height nut to match the bolt strength as this is designed so that the bolt will normally break before the threads start to strip.
Our BOLTCALC program can determine the thread stripping strength so that the length of thread engagement can be optimised.
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